Inorganic glass has various characteristic properties. Of those, various optical functions, and advantage such as formability with which the glass can be subjected to microprocessing, of the glass have allowed the glass to find use in various applications. Representative examples of the applications include: thin-sheet glasses for use in various image display devices such as a sheet glass for a liquid crystal display device and a sheet glass for a plasma display; glasses for optical parts such as lens parts for use in various optical fibers and optical related products surrounding optical fiber applications, and a cover glass for a solid state imaging device for use in image transfer; powder glasses for constructing fine structures for securing the reliability of various semiconductors and of image display devices such as a PDP; and glass products for building materials such as an exterior wall material made of crystallized glass and an interior wall material made of foam-containing glass.
Upon production of such various glass products, the products must be formed accurately into shapes needed for their applications. In view of the foregoing, techniques have been developed for a new forming method and various processing methods such as an abrasion method. In addition, many improvements and developments have been conducted on glass composition to optimize glass properties in accordance with required functions, and a large number of inventions have been made in association with the improvements and developments. In addition, the degree of uniformity in glass composition and the degree of inclusions of bubbles and stones in glass are recognized as homogeneity of glass that is also important in production as well as an article shape and a glass composition.
The measures of the degree of homogeneity of glass depend on the dimensions of a glass structure to which attention is paid, and the dimensions vary depending on the applications of the glass and required quality. In addition, the measures are roughly classified into three scales from an academic point of view. Of those, a measure referred to as a short-range order in dependence on the orientation direction of atomic arrangement (also referred to as a short-range structure) is the smallest measure. Next, a medium-range order is constituted by combining the short-range orders. In addition, a long-range order larger than the medium-range order corresponds to a distance in excess of 1 nm. When a glass structure is interpreted as a short-range order or a medium-range order, any kind of glass has its own structural coordination of elements in dependence on its composition, so it is hard to evaluate the glass by means of the concept of homogeneity in the structure of the glass. Therefore, the concept of homogeneity is applicable to the case where a dimension larger than 1 nm in excess of a medium-range order is taken into consideration. The concept of homogeneity can apply to express a glass state in a scale that is larger than a dimension where a disorder of glass structure can be generally defined. Although a glass structure that is interpreted as any one of those short- and medium-range structures plays an important role in describing the degree of homogeneity of glass, the concept of homogeneity in a larger range is most important in the stage of mass production of glass on a commercial scale.
For example, in such production stage on a commercial scale, the presence of a gas phase in a liquid phase (so-called the presence of bubbles in glass) is a major cause for a significant deterioration in degree of homogeneity of the glass. In addition, even when the problem of bubbles can be avoided, depending on glass composition, a devitrification phenomenon in association with the crystallization from molten glass may occur owing to thermal history, or a dissimilar glass phase having two or more different compositions is obtained. That is, attention must also be paid to a phenomenon referred to as chase separation. Glass defects referred to as a cord, a knot, a stripe, and the like have importance comparable to a degree of homogeneity, and have high risks of spoiling the unction and quality of glass. Those are caused by fluctuations in degree of homogeneity in the long-range order of a glass structure due to the deviation of some specific constituents in a glass composition.
Such heterogeneous sites in glass such as a cord and a knot can be optically detected. Therefore, a degree of optical homogeneity has been defined by representing the quality of glass through the measurement of a refractive index or the like with high accuracy. For example, each of Patent Document 1, Patent Document 2, and Patent Document 3 describes that a degree of homogeneity can be improved by causing such optical refractive index or the like to fall within a specific range.
[Patent Document 1] JP 06-345442 A
[Patent Document 2] JP 10-265226 A
[Patent Document 3] JP 2002-338255 A
However, the degree of optical homogeneity of glass and the degree of homogeneity of the glass composition of the glass do not necessarily coincide with each other. For example, a value for the refractive index of glass used as an indication for the degree of optical homogeneity of the glass can be intentionally changed by adjusting the cooling condition for a formed hot glass article. That is, even when two glasses to be compared with each other have different glass compositions, an appropriate adjustment of a cooling rate can control the refractive indices of the two glasses to be the same. In addition, even when two glasses to be compared with each other have the same composition, an intentional adjustment of a stress steadily acting on each of the glasses can make the two glasses different from each other in refractive index. For example, in actuality, some commercially available optical glasses have the same refractive index and different compositions. Therefore, measuring only the refractive index of each of two glasses to be compared with other does not correspond to a comparison between the degrees of homogeneity of the glasses in terms of composition through measurement, and merely evaluates the glasses only for optical homogeneity.
Meanwhile, it has been requested to have the uniform composition in various high-performance glass products to satisfy multiple difficult demands such as precise dimensional accuracy as well as high chemical durability. For example, when one attempts to form a fine surface shape on a glass article through an etching treatment or the like, the glass surface must be eroded at the same etching rate. This is because a slight deviation in glass composition caused by a reduction in degree of homogeneity affects the amount of a surface to be eroded by etching per unit time, thereby resulting in an inferior surface quality of a glass article. To avoid such circumstance, with a view to realizing a high level of homogeneity of composition, the measurement of multiple physical properties in addition to the above refractive index has been performed to complement the limitation of the precise analysis of glass composition realized by means of a chemical analysis method.
In addition, a tracer method is used as means for confirming a homogeneous production condition at a melting stage or as one promising method of evaluating a flow characteristic in a glass melting furnace. The method involves: adding a trace amount of a metal oxide having a glass coloring effect such as cobalt oxide as a tracer to a glass raw material or mixing lead oxide that can be analyzed in a trace amount as a tracer without any coloring; and evaluating the change of a concentration of such tracer in a glass article with time. Thus, the thermal hysteresis of molten glass inside a glass melting furnace or the degree of mixing due to glass convection, that is, the degree of homogeneity can be detected. However, a glass melting furnace is provided with a small number of openings for preventing heat loss during its operation at high temperature, so it is difficult to incorporate a predetermined amount of tracer into molten glass at an arbitrary position inside the melting furnace. In addition, there arises a problem in that the tracer method cannot be performed very frequently because the method involves the occurrence of the unexpected coloring of a glass product, a change in physical property of glass due to the mixing of a heavy element, or the like.